1. Field of the Invention
The invention relates to a rotating, dynamoelectric machine; and more particularly, to an axial airgap machine comprising two or more stators, wherein the EMF generated in the machine is controlled through the selective rotational alignment of one or more of the stators relative to a reference one of the stators.
2. Description of the Prior Art
The electric motor and generator industry is continuously searching for ways to provide dynamoelectric, rotating machines with increased efficiencies and power densities. As used herein, the term “motor” refers to all classes of motoring and generating machines which convert electrical energy to rotational motion and vice versa. Such machines include devices which may alternatively be called motors, generators, and regenerative motors. The term “regenerative motor” is used herein to refer to a device that may be operated as either an electric motor or a generator. A wide variety of motors are known, including permanent magnet, wound field, induction, variable reluctance, switched reluctance, and brush and brushless types. They may be energized directly from a source of direct or alternating current provided by the electric utility grid, batteries, or other alternative source. Alternatively, they may be supplied by current having the requisite waveform that is synthesized using electronic drive circuitry. Rotational energy derived from any mechanical source may drive a generator. The generator's output may be connected directly to a load or conditioned using power electronic circuitry. Optionally, a given machine is connected to a mechanical source that functions as either a source or sink of mechanical energy during different periods in its operation. The machine thus can act as a regenerative motor, e.g. by connection through power conditioning circuitry capable of four-quadrant operation.
Rotating machines ordinarily include a stationary component known as a stator and a rotating component known as a rotor. Adjacent faces of the rotor and stator are separated by a small airgap traversed by magnetic flux linking the rotor and stator. It will be understood by those skilled in the art that a rotating machine may comprise plural, mechanically connected rotors and plural stators. Virtually all rotating machines are conventionally classifiable as being either radial or axial airgap types. A radial airgap type is one in which the rotor and stator are separated radially and the traversing magnetic flux is directed predominantly perpendicular to the axis of rotation of the rotor. In an axial airgap device, the rotor and stator are axially separated and the flux traversal is predominantly parallel to the rotational axis.
Except for certain specialized types, motors and generators generally employ soft magnetic materials of one or more types. By “soft magnetic material” is meant one that is easily and efficiently magnetized and demagnetized. The energy that is inevitably dissipated in a magnetic material during each magnetization cycle is termed hysteresis loss or core loss. The magnitude of hysteresis loss is a function both of the excitation amplitude and frequency. A soft magnetic material further exhibits high permeability and low magnetic coercivity. Motors and generators also include a source of magnetomotive force, which can be provided either by one or more permanent magnets or by additional soft magnetic material encircled by current-carrying windings. By “permanent magnet material,” also called “hard magnetic material,” is meant a magnetic material that has a high magnetic coercivity and strongly retains its magnetization and resists being demagnetized. Depending on the type of motor, the permanent and soft magnetic materials may be disposed either on the rotor or stator.
By far, the preponderance of motors currently produced use as soft magnetic material various grades of electrical or motor steels, which are alloys of Fe with one or more alloying elements, especially including Si, P, C, and Al. Most commonly, Si is a predominant alloying element. While it is generally believed that motors and generators having rotors constructed with advanced permanent magnet material and stators having cores made with advanced, low-loss soft materials, such as amorphous metal, have the potential to provide substantially higher efficiencies and power densities compared to conventional radial airgap motors and generators, there has been little success in building such machines of either axial or radial airgap type. Previous attempts at incorporating amorphous material into conventional radial or axial airgap machines have been largely unsuccessful commercially. Early designs mainly involved substituting the stator and/or rotor with coils or circular laminations of amorphous metal, typically cut with teeth through the internal or external surface. Amorphous metal has unique magnetic and mechanical properties that make it difficult or impossible to directly substitute for ordinary steels in conventionally designed motors.
Many applications in the electric motor and generator industry require a machine capable of operating significantly beyond a certain base rotational speed during at least part of its ordinary use. The base speed is the highest value attainable when an electric device is operated in a constant-torque mode. Above the base speed, the back EMF ordinarily exceeds a nominal supply voltage. However, design optimization is challenging for many applications in which the machine must operate at a wide range of speeds. The problem is especially acute for systems which do not incorporate a variable-ratio gearbox or other speed-matching device. For example, low-speed operation in an electric vehicle often requires constant-torque operation for moving heavy loads or traversing rough terrain or inclines, such as mountain trails, which normally are done at much less than a base speed. However, high-speed operation, e.g. for cruising on level roads or developed industrial sites, may require double or triple the base speed. For high-speed operation, torque requirements are generally low, and constant power operation, wherein the available torque is inversely proportional to the speed, would afford significant advantages.
A recognized disadvantage of typical permanent magnet machines is that the generated EMF of the machine is a direct linear function of the rotational speed of the machine. The generated EMF is also directly proportional to power output for a given current. Although greater power can be obtained at higher speeds, higher voltages are concomitantly produced during generating applications. Similarly, in motoring applications, the power supply voltage must be increased to go above the voltage need at the base speed. In either case, construction techniques and materials, particularly including insulation, and semiconductor and electronic elements in the control circuitry, must be selected accordingly. As a result, higher voltages are difficult if not impossible to control cost effectively. Thus a controlled and controllable generated EMF is a desirable feature in a machine, since speed limitations can be relaxed.
Prior art references have taught methods of maintaining a constant terminal voltage during operation of electric devices, based on manipulating the airgap between the rotor and the stator. A small decrease in the airgap results in an increase in the voltage (EMF) generated in the stator windings, and vice versa. U.S. Pat. Nos. 2,892,144 and 2,824,275 disclose a generator comprising a single stator positioned opposite a rotor, wherein the stator is mounted such that an increase in torque during operation ultimately causes motion of the stator towards the rotor, i.e., tending to reduce the airgap. An increased load (torque) that would ordinarily result in a drop in output voltage also causes a reduction in the airgap, which results in an increase in the voltage.
In an alternative embodiment, U.S. Pat. No. 2,824,275 discloses a generator comprising a single fixed stator positioned opposite a rotor, wherein the rotor is mounted such that an increase in speed during operation ultimately causes motion of the rotor away from the stator, i.e., tending to increase the airgap. As the output voltage is proportional to the speed, increasing speed would result in increasing voltage. However, an increasing airgap acts to reduce the voltage.
As another example of a manipulation of the airgap of a different type of electric device, U.S. Pat. No. 5,627,419 discloses a modified radial airgap flywheel with self-engaging means for automatically decreasing the adjustable airgap between the stator and the flywheel in response to electromagnetic torques exerted on the stator during spin-up or spin-down, as well as for increasing the adjustable airgap during freewheeling operation.
Other methods are known for controlling output parameters of electric devices during operation through manipulating the overlap between the rotor and the stator in radial airgap machines. As a method of maintaining constant speed during operation, U.S. Pat. No. 403,017 discloses using the centrifugal force on governors attached to the rotor of a radial airgap motor to reduce the axial overlap between the rotor and stator. A reduction of the load on the motor would normally result in an increase in the speed, but the increase in speed increases the centrifugal force on the governors, which causes an axial displacement of the rotor relative to the stator, thus reducing the overlap between rotor and stator. The reduced overlap between the rotor and stator results in reduced torque, which then counteracts the tendency for the increasing speed.
More recently, U.S. Pat. No. 6,555,941 discloses a method of reducing the back EMF of a radial airgap motor by axially displacing the rotor relative to the stator, hence reducing the overlap. As the rotor is offset into greater axial misalignment with the stator, the magnet flux on the stator field coils is reduced, thus reducing the back EMF that limits the speed. With the rotor misaligned, the motor operates in constant power mode, where the available torque is inversely proportional to the speed.
U.S. Pat. No. 6,194,802 also discloses a method of reducing the back EMF by reducing the overlap between the rotor and stator in an axial airgap motor. The rotor magnet blocks are mounted on the rotor such that an increase in speed during operation results in an increase in centrifugal force on the magnet blocks, causing them to move outwards from the center of the motor. This outwards motion results in a reduction in the overlap between the magnet block and the stator, thereby reducing flux linkage and the back EMF generated. Accordingly, the machine can rotate at higher speeds.
High speed (i.e., high rpm) electric machines are almost always manufactured with low pole counts, lest the magnetic materials in electric machines operating at higher frequencies experience excessive core losses that contribute to inefficient motor design. This is mainly due to the fact that the soft material used in the vast majority of present motors is a silicon-iron alloy (Si—Fe). It is well known that losses resulting from changing a magnetic field at frequencies greater than about 400 Hz in conventional Si—Fe-based materials causes the material to heat, oftentimes to a point where the device cannot be cooled by any acceptable means. A number of applications in current technology, including widely diverse areas such as high-speed machine tools, aerospace motors and actuators, and compressor drives, require electrical motors operable at high speeds, many times in excess of 15,000–20,000 rpm, and in some cases up to 100,000 rpm.
To date it has proven very difficult to cost effectively provide readily manufacturable electric devices, which take advantage of low-loss materials. Previous attempts to incorporate low-loss materials into conventional machines generally failed, since the early designs typically relied on merely substituting new soft magnetic materials, such as amorphous metal, for conventional alloys, such as silicon-iron, in machine's magnetic cores. The resulting electric machines have sometimes provided increased efficiencies with less loss, but they generally suffer from an unacceptable reduction in power output, and significant increases in cost associated with handling and forming the amorphous metal. As a result, they have not achieved commercial success or market penetration.
Thus, there remains a need in the art for highly efficient axial airgap electric devices, which take full advantage of the specific characteristics associated with low-loss material, thus eliminating the disadvantages associated with conventional axial gap machines. Ideally, an improved machine would provide higher efficiency of conversion between mechanical and electrical energy forms. Improved efficiency in generating machines powered by fossil fuels would concomitantly reduce air pollution. The machine would be smaller, lighter, and satisfy more demanding requirements of torque, power, and speed. Cooling requirements would be reduced. Motors operating from battery power would operate longer. In addition, there remains a need for devices that can operate efficiently in either constant torque mode, or, with suitable back EMF control, in constant power mode. Further desired are machines in which torque ripple and cogging, and concomitant electrical ripple, are reduced, e.g. by increased pole count.